46o RESEARCHES ON FUNGI 



The problem why it is that a discharged sporangium normally lands 

 on a grass-leaf, etc., so that its under gelatinous side is turned toward 

 the surface of the substratum and its black upper convex side away 

 from the surface of the substratum has been solved by taking into account 

 the fact that (1) the lower gelatinous side of the sporangium can be 

 wetted by water whereas (2) the upper convex side covered by the black 

 sporangial wall is unwettable. That the black sporangial wall is un- 

 wettable has been demonstrated by various experiments. A Pilobolus 

 projectile, whilst travelling through the air, consists of a sporangium 

 and a large drop of cell-sap. The drop does not enclose the sporangium 

 but is attached to the sporangium's wettable gelatinous under side. 

 The projectile may rotate as it passes through the air. As the projectile 

 lands on a grass-leaf, etc., the drop spreads out on the surface of the 

 substratum. As it does so, it drags the gelatinous wettable under side 

 of the sporangium toward the surface of the substratum and, at the 

 same time, pushes away from itself and the substratum the black 

 unwettable convex side of the sporangium. 



Very occasionally one observes a sporangium which is attached to 

 the surface of glass, etc., not by its gelatinous under side, but by its 

 rounded black upper side. The taking up of this abnormal position by 

 a sporangium may be due to the gelatinous ring having been torn away 

 from the sporangium before the projectile struck the surface of the 

 substratum or at the moment of impact. 



The gelatinous material on the under side of a sporangium has 

 remarkable adhesive properties, as may be shown by attempts to wash 

 sporangia off sheets of glass, etc., to which they have become attached. 

 Under natural conditions in pastures, the sporangia of Pilobolus are so 

 effectively attached by their gelatinous sides to the herbage on which 

 they have alighted that, in dry weather, they cannot be detached from 

 their substratum by the strongest winds and, in wet weather, they cannot 

 easily be detached by prolonged rain or even by violent thunderstorms. 



Pilobolus is a highly specialised coprophilous fungus which is dependent 

 for its existence : (1) on flowering plants which provide its sporangia 

 with a temporary but prospectively favourable lodging-place ; and (2) on 

 herbivorous animals which swallow the sporangia and herbage together, 

 break open the sporangia and disperse the spores within their alimentary 

 canals, and finally extrude the spores undamaged in their solid faeces. 



The sporangial wall of Pilobolus, as opposed to that of Mucor, is 

 (1) unwettable, (2) persi.stent (non-deliquescent), and (3) intensely black. 

 The unwettability of the wall is a prime factor in causing the sporangium 

 to alight on any object with its gelatinous side turned toward the object ; 

 the persistency of the wall enables the wall, when the sporangium is 

 attached for weeks or months to a flowering plant, to prevent the spores 

 escaping from the sporangium even during rainy weather ; and the 

 intensely black pigment in the wall enables the wall to absorb sunlight 



